What distinguishes Class IV from Class III cavities in Black's Classification?

Class IV cavities are characterized by their involvement of the incisal edge, which is the biting surface of anterior teeth, specifically incisors and canines. This type of cavity typically affects the proximal surfaces, which are the sides of the teeth that contact adjacent teeth, and it also affects the incisal edge, making it distinct from Class III cavities that only involve the proximal surfaces but do not extend to the incisal edge.

The focus on the incisal edge is significant because it often requires different treatment considerations, such as restorative techniques to match the aesthetics of the tooth and to ensure proper function. Therefore, the defining feature that sets Class IV apart is this involvement of the incisal edge, which is not a factor for Class III cavities. This distinction is crucial for dental professionals when diagnosing and planning treatment for carious lesions.